The invention relates to a process for producing a ceramic body, in particular a polychromatically, monochromatically or spatially colored dental ceramic blank, with selectively adjustable degrees of expression of one or more physical properties. The one or more physical properties preferably exhibit varying degrees of expression with respect to spatially different regions of the ceramic body, or the ceramic body exhibits a planar, i.e. two-dimensional, and/or a spatial, i.e. three-dimensional, progression of the degrees of expression of the one or more physical properties. The physical properties are understood to include, for example, optical properties, such as opacity and translucency, mechanical properties, such as hardness, (flexural) strength, and (fracture) toughness, and structural properties, such as crystal system configurations, density and/or porosity, etc.
In recent years, what is known as yttrium stabilized zirconium oxide, more particularly, partially stabilized zirconium oxide interspersed with approximately a 3% mole fraction of yttrium oxide (3Y-TZP=yttria-tetragonal zirconia polycrystals), has gained widespread use as an all-ceramic material in the field of dental technology. This is due primarily to the high mechanical stability and thermal resistance of these high-performance ceramic frameworks, but also to their outstanding biocompatibility. Coloring of all-ceramic dental prostheses is typically carried out in a final step by means of veneering, i.e. the application of an additional ceramic layer onto the dental prosthesis, such as a crown or bridge, which has already been shaped through milling. These additional ceramic layers, which possesses outstanding coloring and shaping possibilities, are still being applied by hand on an individual basis, and with great effort, to each individual dental prosthesis. However, studies have shown that the chipping rate of this aesthetic ceramic veneer is five times as great as that of established metallic ceramic veneering systems (VMK). Complaints associated with aesthetic ceramic veneers, along with high manufacturing costs, make it desirable to dispense with these veneers. However, without ceramic veneers, the hardness of yttrium-stabilized zirconium oxide full ceramics is twice as high as that of a natural tooth crown, which leads to a wearing down of opposing and neighboring teeth and thus to damage of the remaining dentition.
Another problem that results when a ceramic veneer is dispensed with is the optical or aesthetic appearance of the dental prosthesis. It should be noted here that no unicolored tooth exists in the mouth of any patient. In a natural tooth, the darker dentine core is covered by ever-thickening layers of cutting edge mass. This leads to varying coloration and light transmission properties of the natural tooth, all the way to the virtually transparent cutting edges. A traditional, non-veneered ceramic dental prosthesis, in particular one made of pure 3Y-TZP, exhibits a spatially uniform expression of color and physical properties, in particular of hardness, but also of light transmission properties such as translucency or opacity.
To enable a crown or, alternatively, the dental prosthesis to function on an aesthetic level, the following features must be considered and implemented. To begin with, the layer of dentine must have the basic tooth color of the patient. The dentine color and the thickness of the cutting edge layer create the actual tooth color of the patient. The natural tooth develops the three-dimensional color combination during growth and through later wear and tear. In addition, there is an ever-lightening color progression starting from the dentine core all the way to the cutting edge. The dental enamel may also exhibit lighter and/or transparent areas. Furthermore, in older patients the darker dentine-colored edges of crowns are clearly visible, and therefore the new dental prostheses must be adapted accordingly.
Various processes for coloring dental prostheses with metallic ions and/or metal complexing solutions and gels are known from WO 2008 098 157, WO 2013 055 432, WO 0046 168, WO 2004 110 959, DE 199 04 522 B4, DE 10 2008 026 980 A1, WO 00/46168 A1, WO 2011/156602 A2 or DE 20 2011 109 956 U1, WO 11 15 66 02, EP 2013 06 31 20, and US 2011 039 805. However, at most a simple and unicolored coloration of a dental prosthesis can be achieved through immersion or spraying. Although attempts have been made to achieve tooth color structures or similar colorations of a dental prosthesis using brushes or by applying drops, it is nevertheless impossible to achieve natural-looking results with known fluids or with known solutions or gels, and as a result, noticeable color differences persist between natural teeth and dental prostheses.
To be able to produce dental prostheses in an automated process, rather than producing them manually on an individualized basis, it would be necessary, for example, to produce a multitude of very different ceramic blanks with spatial color progressions but also with spatial progressions of physical properties, if economical and aesthetic results are to be achieved.
The patents EP 202 4300, WO 2014 062 375, WO 02 09 6 12, U.S. Pat. No. 9,212,065 B2, DE 2020 090 187 24, EP 235 97 71 and EP 185 97 57 teach the coloring of the starting material, in particular of powders or pastes. The powders or pastes are then poured or applied in layers, with each layer exhibiting a specific color. Thus 7-10 layers are required in order to achieve a two-dimensional coloring or alternatively to achieve a two-dimensional or planar color progression.
A process for producing a ceramic dental prosthesis with improved optical translucency is known from US 2011 269 618. The starting material, tetragonal polycrystalline zirconium powder, is developed for this purpose with a particularly small particle size. Due to the smaller particle size, the dental prosthesis that is pressed from the material has a lower refractive index and therefore increased translucency. According to US 2013 022 15 54 A1, it is possible to achieve at least a roughly graduated progression of optical physical properties such as opacity and/or translucency, in a manner that is similar to the coloring of dental ceramic blanks, through the layered pouring of various ceramic dental powders that have varying yttrium content. The pressed dental ceramic blank then exhibits a roughly graduated two-dimensional progression of opacity, which corresponds to the number and the thickness of the layers.
One disadvantage of the aforementioned processes is that only a planar, i.e. a two-dimensional color progression and/or progression of the degree of translucency can be achieved in the manner described. Beyond this, the creation of each additional gradation of the color progression and/or progression of expression requires an additional process step. The production of a three-dimensional or spatially as well as finely graduated progression would require the pouring of a multitude of powder layers not only on top of one another but also side by side, which would be beyond the scope of the process described and which would lead to disproportionate additional costs. Amongst other things, it would be necessary to store, test and inspect hundreds of different powders with different physical properties and colorings.
A complicated sol-gel process for producing a millable dental ceramic blank with geometrically defined areas of differentially expressed translucency is known from US 2015 028 2905 A1. In the same, a first zirconium solution is cast into a mold and is cured to form a first zirconium gel. The first zirconium gel is placed in a second, larger mold and is overmolded by a second zirconium solution. The process is then repeated until the desired number of zirconium gel layers is achieved. The layered zirconium gel body is subsequently sintered, to obtain a millable dental ceramic blank. In this way, each zirconium layer can exhibit a different translucency. While it is true that the described process can also be used to achieve spatially differentiated degrees of translucency within the dental ceramic blank, each additional region nonetheless requires an additional process step that involves casting the zirconium solution and then curing the solution to form the zirconium gel, as a result of which the process is exceptionally time-consuming and would thus be uneconomical.